Field Marshal Paul von Hindenburg is considered Germany's great military hero of WW I. Asprey ( War in the Shadows ) here confirms the theory that his chief of staff, Gen. Erich Ludendorff, was the brains behind Hindenburg's victories and largely responsible for German military policy in the war's latter years. The Duo, as the author calls them, became increasingly involved in political affairs, replacing imperial chancellor Theobald Hedwig (whom they regarded as a defeatist) with the unknown politician Georg Michaelis (whom they counted on to do their bidding) and imposing the harsh treaty of Brest-Litovsk on the Russians. Ludendorff later fled, in disguise, to Sweden, while Hindenburg led his troops home after the 1918 armistice, declaring that the German army had not been beaten but betrayed--the origin of the ``stab in the back'' concept that became Adolf Hitler's rallying cry. The double biography sheds new light on Germany's conduct in WW I and on the character of the legendary General Staff. Most interestingly, Asprey seeks to explain Germany's disastrous defeat as a result of ``expanded military egos unchecked by civil authority.'' Photos. (Dec.)